1. Field of the Invention
The invention relates to the extrusion of resinous thermoplastic materials, and more particularly to the method for extruding biaxially oriented high density polyethylene film in which an additional secondary gaseous cooling medium is applied at strategic strain inducing locations of the blown bubble to control the final product properties of the extruded material and to the product produced by such method.
2. Brief Description of the Prior Art
It is well known to use primary air against a blown bubble of plastic to aid in the expansion of the bubble and the formation of such bubble into the desired shape.
In the normal high or medium molecular weight straight chain resin extrusion to ensure adequate orientation, small die sizes (31/2", 4", 6", 8") are used with one cooling air (primary) placed at the die top to cool the outgoing melt. Melt is drawn at about 1:1 blow-up-ratio (B.U.R.) to about 6 times the die diameter in height to give machine direction (MD) orientation and then blown in transverse direction to required lay flat width 3.2 to 3.5 B.U.R. to give adequate transverse direction (TD) orientation. It is well known when properties of these films, i.e., from 3.5" to 8" die sizes are tabulated they follow a descending order. In other words, total orientation MD/TD from a 31/2" or 4" die is far superior to an 8" die. Following is an example of resin having a 0.954 density and 0.06 melt index film properties from a 4" and a 6" die using a normal blown process, i.e., one primary air cooling at die top, no internal drag.
______________________________________ Die Size 4" 6" Long Flat 20" tubes 40" (2 up 20" tubes) ______________________________________ Gauge mils 0.7 0.7 B.U.R. 3.2 4.2 Yield tensile psi MD 5300 4300 TD 5500 3600 MD/TD (0.96) (1.19) Ultimate tensile psi MD 8900 5900 TD 8200 3800 MD/TD (1.08) (1.55) Elongation % MD 476 395 TD 354 540 MD/TD (1.34) (0.73) Tear gms/mil MD 33 16 TD 158 397 MD/TD (0.2) (.04) TD/MD 5 25 Impact gms/mil 280 68.5 Output 150 lbs/hr. 220 lbs./hr. Neck height inches 24" 38" ______________________________________
Like any other industry, for a good return on investment, i.e. high output, multiple bags must be produced in-line. Present technology does not enable us to achieve this with good or optimum product properties using a larger die size. Present technology uses small dies that give small lay flat widths (1 up) and at a lower output. On the other hand, the latest bag machines are designed to run 3 up bags. Hence matching of extruder to bag machine now becomes a problem.
The use of a secondary cooling source for a low density polymer extrusion to upgrade optical properties (haze and gloss) or to increase output is disclosed in the following patents:
Canadian Pat. No. 922,063--Mar. 6, 1973--St. Eve and Bose PA0 U.S. Pat. No. 3,754,067--Aug. 21, 1973--St. Eve and Bose
Secondary cooling to give more rigidity so that a longitudinal continuous slitting can be done (sheeting out of a tube) is disclosed in U.S. Pat. No. 4,115,048, Sept. 19, 1978, Alderfer.
The present invention differs drastically in approach from such prior art.
The method of the present invention employs secondary cooled air from a ring which can be moved upward and downward along the vertical axis of the bubble to direct air at the point where transverse direction orientation forces take over. This differentiates it from Canadian Pat. No. 922,063. The cooling air is delivered from a straight nozzle or without a nozzle, without a chimney, thus having normal air with a venturi effect on the bubble. It has been found that such secondary cooled air to control the strain rate permits control of the final film properties, e.g. tear, impact and balancing of tensile, elongation, etc. The optimization of product properties in this manner is not found in the prior art for the conventional extrusion of resinous thermoplastic materials.